1. Field of the Invention
The present invention relates generally to a radio frequency (RF) transceiver of a Time Division Duplexing (TDD) wireless communication system, and in particular, to an apparatus for protecting a Low Noise Amplifier of a receiver block.
2. Description of the Related Art
Generally, TDD wireless communication systems separate a transmission and a reception by time division of the same frequency. In the TDD wireless communication system, a transmit/receive antenna switch (TRAS) performs a switching function between a high power RF transmit (TX) signal and a low power RF receive (RX) signal. The TRAS protects a low noise amplifier (LNA) by interrupting a transmission power supplied to a receiver in a TX mode and reduces noise introduced from a transmitter in an RX mode.
FIG. 1 is a block diagram of an RF transceiver in a conventional TDD wireless communication system. Referring to FIG. 1, the RF transceiver includes a TDD controller 100, a transmitter block (TXB) 110, a power amplifier block (PAB) 120, a receiver block (RXB) 130, an LNA 140, a TRAS 150, a front end block (FEB) 160, and antenna 170.
The TDD controller 100 generates TDD control signals to the respective blocks according to TX/RX modes. In FIG. 1, a reference symbol “TCTX” represents a TDD control signal for the TXB 110 and will be referred to as a first control signal. A reference symbol “TCPA” represents a TDD control signal for the PAB 120 and will be referred to as a second control signal. A reference symbol “TCAS” represents a TDD control signal for the TRAS 150 and will be referred to as a third control signal. Also, a reference symbol “TCRX” represents a TDD control signal for the RXB 130 and will be referred to as a fourth control signal.
In the TX mode, the TXB 110 converts a baseband modulated signal input from a modem (not shown) into an RF signal according to the first control signal TCTX, and the PAB 120 power-amplifies a TX signal received from the TXB 110.
The TRAS 150 transmits a signal from the PAB 120 to the FEB 160 in the TX mode, and transmits an output signal of the FEB 160 to the LNA 140 in the RX mode. In the TX mode, the TRAS 150 protects the receiver circuit from a high power TX signal by isolating the receiver circuit from a TX path. In the RX mode, the TRAS 150 blocks a noise signal transmitted from the PAB 120 by isolating the transmitter circuit from an RX path. The TRAS 150 may be implemented using an RF switch or a circulator.
The FEB 160 includes a band pass filter (BPF). The FEB 160 performs a service band filtering operation on a TX signal received from the TRAS 150, and outputs the filtered TX signal through the antenna 170. Also, the FEB 160 performs a service band filtering operation on an RX signal received from the antenna 170, and outputs the filtered RX signal to the TRAS 150.
The LNA 140 amplifies the signal output from the TRAS 150 while suppressing its noise. In the RX mode, the RXB 130 converts an RF signal received from the LNA 140 into a baseband signal according to the fourth control signal TCRX, and outputs the baseband signal to the modem.
FIG. 2 illustrates a relationship between the TDD control signal and the TX/RX RF signal.
Referring to FIG. 2, the third control signal TCAS for controlling the operation of the TRAS 150 maintains a high state during a transmit time (TT) period and changes to a low state at a time point when the TT period is ended. Next, the third control signal TCAS maintains a low state during a receive guard time (RGT) period and a receive time (RT) period and changes into a high state at a time point when the RT period is ended. Then, the third control signal TCAS maintains a high state during the transmit guard time (TGT) period and the TT period and changes to a low state at a time point when the TT period is ended.
The TGT period is a guard time necessary for the blocks of the system to perform mode switching when the system switches from the RX mode to the TX mode. The RGT period is a guard time necessary for the blocks of the system to perform mode switching when the system switches from the TX mode to the RX mode.
The high power RF TX signal is transmitted while the third control signal TCAS is in the high state. On the other hand, the low power RF RX signal is transmitted while the third control signal TCAS is in the low state.
To operate the TDD system normally, the blocks of the system have to complete mode switching within the TGT period and the RGT period. All the blocks on the TX path have to maintain the normal state during the TT period, and all the blocks on the RX path have to maintain the normal state during the RT period. That is, to achieve the normal path switching of the TDD system, the TDD control signals for controlling the respective blocks have to be in the normal state, the blocks on the TX path have to switch to the TX mode within the TGT period, and the blocks on the RX path have to switch to the RX mode within the RGT period. In addition, the TRAS 150 has to secure a sufficient isolation level between the TX path and the RX path.
In mode switching, the blocks constructing the TX path and the RX path in the TDD system of FIG. 1 have different mode switching times and delay times of the RF signal. Therefore, the TDD controller 100 has to generate the TDD control signals TCTX, TCPA, TCAS and TCRX, considering the mode switching time of the respective blocks and the delay time of the RF signal.
However, when there occur problems in the TDD control signals generated from the TDD controller 100, or when there occur problems in the control connection between the TDD controller 100 and a corresponding block, the isolation degree between the high power TX signal and the highlow power RX signal cannot be sufficiently secured by the asynchronous operation of the blocks. In this case, the receiver block, especially the LNA 140, may be damaged by the high power RF TX signal.
FIG. 3 illustrates the high power RF TX signal that is introduced into the RX path when the third control signal TCAS is abnormal.
Referring to FIG. 3, the PAB power-amplifies an input signal while the second control signal TCPA is in a high state, but does not perform the amplification operation while the second control signal TCPA is in a low state. When the third control signal TCAS is in a high state, the TRAS 150 operates in the TX mode to transmit the output of the PAB 120 to an antenna feed line, and isolates the RX mode from the TX mode. On the other hand, when the third control signal TCAS is in a low state, the TRAS 150 operates in the RX mode to transmit signals supplied through the antenna feed line to the LNA 140.
If the high power RF signal from the PAB 120 is transmitted to the TRAS 150 during the RT period of the third control signal TCAS because the TT period and the RT period of the second and third control signals TCPA and TCAS do not coincide with each other, the TRAS 150 does not operate in the RX mode. Therefore, the high power RF signal is introduced into the LNA 140. In this case, the LNA is permanently damaged.
In practice, these problems often occur when the system is initially installed, the blocks are replaced or additionally installed so as to extend frequency allocation (FA), or the blocks are assembled/disassembled for their inspection. Therefore, there is a demand for an apparatus that can protect the receiver circuit even in an abnormal state.